Fungus Study Guide

📖 Core Concepts Fungus – a eukaryotic kingdom (Fungi) whose cells have chitin‑β‑glucan cell walls and the sterol ergosterol in the plasma membrane. Nutrition – obligate heterotrophs; obtain carbon/energy by absorbing dissolved organic molecules (no photosynthesis). Hyphal growth – filamentous cells extend at the tip; the Spitzenkörper (vesicle‑rich body) directs vesicle delivery for cell‑wall synthesis. Hyphal types – septate (cross‑walls with pores) vs coenocytic (no septa). Basidiomycota have a characteristic dolipore septum. Dikaryotic phase – after plasmogamy, two nuclei coexist in each cell (common in Ascomycota & Basidiomycota) before karyogamy. Sexual cycle – plasmogamy → (optional dikaryotic phase) → karyogamy → meiosis → spores. Asexual reproduction – conidiophores → conidia; mycelial fragmentation → clonal colonies; Deuteromycota = fungi with no known sexual stage. Mycorrhizal symbiosis – >90 % of plants partner with fungi (arbuscular or ectomycorrhizae) to obtain N, P, water; hyphae form a common mycorrhizal network (“underground internet”). Appressoria – swollen infection cells that build turgor pressure > 8 MPa to mechanically pierce plant surfaces. Phylogeny – fungi belong to Opisthokonta, closer to animals than plants; seven phyla (Chytridiomycota, Blastocladiomycota, Zoopagomycota, Mucoromycota, Glomeromycota, Ascomycota, Basidiomycota). --- 📌 Must Remember Fungal cell wall = chitin + β‑glucans + mannoproteins. Ergosterol = hallmark sterol; target of many antifungals. Septate hyphae → pores; coenocytic hyphae → no septa. Dolipore septum → diagnostic of Basidiomycota. Clamp connections (Basidiomycota) vs croziers (Ascomycota) maintain dikaryons. Heterothallic = need two compatible mating types; homothallic = self‑fertile. Appressorial pressure ≈ $>8\ \text{MPa}$; key for plant‑pathogenic fungi (e.g., Magnaporthe oryzae). Spore dispersal – most spores are dry & hydrophobic → wind transport. Mycorrhizal benefit – ↑ plant nutrient uptake, carbon sequestration, soil aggregation. Major phyla traits: Chytridiomycota – flagellated zoospores. Glomeromycota – arbuscular mycorrhizae, asexual. Ascomycota – ascus → ascospores; many yeasts. Basidiomycota – basidium → basidiospores; mushrooms. --- 🔄 Key Processes | Process | Step‑by‑step outline | |---------|----------------------| | Sexual reproduction (general) | 1. Plasmogamy – fusion of two compatible hyphae.<br>2. Dikaryotic phase (optional) – nuclei coexist, migrate with clamp connections (Basidiomycota) or croziers (Ascomycota).<br>3. Karyogamy – nuclear fusion in the ascus or basidium.<br>4. Meiosis → haploid spores (ascospores or basidiospores). | | Asexual conidium formation | 1. Hyphal tip differentiates into a conidiophore.<br>2. Phialides (bottle‑shaped cells) produce conidia by budding.<br>3. Conidia are released → wind/water dispersal. | | Appressorium‑mediated infection | 1. Germ tube contacts host surface.<br>2. Differentiates into an appressorium.<br>3. Accumulates osmolytes → builds turgor > 8 MPa.<br>4. Generates a penetration peg that punctures the cuticle. | | Mycorrhizal nutrient exchange | 1. Fungal hyphae colonize root cortex (intracellular arbuscules or extraradical mantle).<br>2. Plant supplies carbon (sugars) to fungus.<br>3. Fungus transfers inorganic nutrients (P, N, water) to plant via arbuscule exchange interface. | | Spore dispersal (wind) | 1. Production of dry, hydrophobic spores.<br>2. Release aided by ballistospore catapult (surface tension) or simply by gravity.<br>3. Air currents carry spores → colonize new substrates. | --- 🔍 Key Comparisons Ascomycota vs Basidiomycota Ascus (sac) → ascospores vs basidium (club) → basidiospores. Croziers maintain dikaryons vs Clamp connections. Mostly septate hyphae; Basidiomycota have dolipore septa. Septate vs Coenocytic hyphae Septate = cross‑walls with pores (allow cytoplasmic streaming). Coenocytic = no septa → multinucleate cytoplasm. Heterothallic vs Homothallic Heterothallic = require two compatible mating types. Homothallic = self‑compatible, can complete sexual cycle alone. True fungi vs Oomycetes (water molds) Cell wall: chitin + β‑glucan vs cellulose. Phylogeny: Opisthokonta vs Stramenopiles. Arbuscular mycorrhizae vs Ectomycorrhizae Arbuscular: intracellular arbuscules (Glomeromycota). Ectomycorrhizae: mantle & Hartig net around root tips (mostly Basidiomycota). --- ⚠️ Common Misunderstandings “Fungi are plants.” – They lack chlorophyll, have chitin walls, and are more closely related to animals. All fungi have septate hyphae. – Many (e.g., Mucoromycota) are coenocytic. Appressoria only use mechanical force. – High osmotic turgor + enzymatic degradation act together. All molds are Ascomycota. – Some belong to Basidiomycota or even Deuteromycota (imperfect). Oomycete spores are fungal. – Oomycetes have cellulose walls and belong to a different kingdom. --- 🧠 Mental Models / Intuition Fungi as “chemical sponges.” – Imagine hyphae as endless tubes that soak up nutrients, guided by the Spitzenkörper “assembly line.” Mycorrhizal network = internet. – Carbon and nutrients travel like data packets between plants through hyphal “cables.” Dikaryon = two‑person conversation. – Two nuclei share the same cytoplasm, “talk” before finally fusing (karyogamy). Appressorium = pressurized syringe. – Build internal pressure >8 MPa, then inject a penetration peg into the host. --- 🚩 Exceptions & Edge Cases Chytridiomycota – produce flagellated zoospores (unique among fungi). Glomeromycota – reproduce only asexually (no known sexual spores). Zygomycota / Mucoromycota – form a thick‑walled zygospore after gametangial fusion. Deuteromycota (Imperfect fungi) – no observed sexual stage; placed by morphology only. Some Basidiomycota lack clamp connections (e.g., certain rust fungi). --- 📍 When to Use Which Identify phylum See flagellated spores → Chytridiomycota. Presence of arbuscules in roots → Glomeromycota. Dolipore septa or clamp connections → Basidiomycota. Asci with croziers → Ascomycota. Choose control strategy Biocontrol (entomopathogenic fungi) for insect pests. Fungicides targeting ergosterol synthesis for human/plant pathogens. Crop rotation or resistant cultivars for soil‑borne Mucoromycota rot. Select laboratory media Potato dextrose agar → general mold growth & colony morphology. Selective media with antibiotics → isolate yeasts from mixed samples. --- 👀 Patterns to Recognize Dry, hydrophobic spores → wind dispersal; typical of many Ascomycota & Basidiomycota. High turgor appressoria + melanized cell wall → aggressive plant pathogen (e.g., Magnaporthe). Dolipore septa under microscope → Basidiomycota. Croziers at ascus base → Ascomycota. Arbuscule‑filled cortical cells → Glomeromycota mycorrhiza. Coenocytic hyphae with no visible septa → Mucoromycota/Zygomycota. --- 🗂️ Exam Traps “All fungi have chitin, so any organism with chitin is a fungus.” – Oomycetes have cellulose, but some protists also have chitin. Confusing basidiospores with conidia – basidiospores are sexual, produced on basidia; conidia are asexual. Assuming all molds are Ascomycota – many Basidiomycota (e.g., Coprinus) form mold‑like colonies. Mix‑up of heterothallic vs homothallic – remember heterothallic = requires a partner; homothallic = self‑compatible. Believing mycorrhizal fungi are always beneficial – some can become myco‑heterotrophic parasites under certain conditions. Treating Deuteromycota as a true taxonomic group – they are a “catch‑all” for fungi with unknown sexual stages, not a phylogenetic clade. ---